An integrated circuit (IC) chip often includes core devices for performing certain functions and input/output (I/O) devices for communicating with external circuits. For example, a metal-oxide-semiconductor (MOS) transistor may be implemented as either a core device, or an I/O device.
Transistors used for core devices and I/O devices are often different. For example, an I/O device usually has a relatively thick gate oxide in order to sustain a high voltage applied to the peripheral circuit. On the other hand, a core device often requires a very thin gate oxide to achieve a strong capacitive effect for good current control. However, on an IC chip, core devices and I/O devices are usually fabricated at approximately the same level relative to the substrate, which sometimes makes it challenging to make the core devices and the I/O devices according to their respective best specifications. For example, the core devices often suffer leakage currents to the substrate.